Geological Hazards

Mass movement is the downslope movement of rock, debris or soil under gravity. Varnes (1978) classified landslides by the type of movement and the type of material (rock, debris, earth/soil).

Types of movement

1. Fall: detachment and free fall, bouncing or rolling of material from steep slope; rock fall, debris fall.
2. Topple: forward rotation of rock mass about a point below its centre of gravity; columnar jointed rock.
3. Slide: movement of mass along a surface of rupture.
   • Rotational slide (slump): curved slip surface, in homogeneous soil.
   • Translational (planar) slide: flat surface, along bedding, joint or foliation. Includes wedge slide.
4. Spread (lateral spreading): lateral extension of a mass with shear or tension fractures; on gentle slopes.
5. Flow: the mass moves as a viscous fluid; debris flow, mud flow, earth flow, rock avalanche.
6. Complex: combination of two or more types, e.g. slide-flow.

Classification table

Rate of movement: from extremely slow (below 16 mm/year) to extremely rapid (above 5 m/s). Water content and speed distinguish slides from flows.

Difference

How strength is measured

1. Richter local magnitude (1935):

$$M_L = \log_{10} A + 3\log_{10}(8\Delta t) - 2.92$$

where $A$ is the maximum amplitude on the Wood-Anderson seismograph in mm and $\Delta t$ is the S-P time lag in s. Each unit increase in magnitude gives a 10 times larger amplitude and about 32 times more energy.

2. Moment magnitude ($M_w$), preferred for large earthquakes:

$$M_w = \frac{2}{3}\log_{10} M_0 - 6.07$$

($M_0$ = seismic moment in N m = rigidity $\times$ fault area $\times$ slip.)

3. Energy relation: $\log_{10} E = 1.5 M + 4.8$ ($E$ in joules); so energy ratio per unit magnitude is $10^{1.5}\approx 31.6$ (checked).

4. Intensity is assigned using the Modified Mercalli scale after field observation (I: not felt; V: felt by nearly all; VII: damage to poor buildings; X: most masonry destroyed; XII: total destruction).

Mass movement (mass wasting) is the downslope movement of rock, debris or soil under the influence of gravity, with or without water as a helping agent. Landslide, rock fall, creep, debris flow and subsidence are all forms.

Mechanism

A slope fails when the driving (shear) forces exceed the resisting forces (shear strength). For a mass of weight $W$ on a slope of angle $\beta$:

$$\text{Factor of safety } F = \frac{\text{Resisting force}}{\text{Driving force}} = \frac{c A + W\cos\beta\tan\phi}{W\sin\beta}$$

Failure begins when $F<1$.

• Increase of driving force: extra load (buildings, fill, rain water), steepening by river cutting or road excavation, removal of toe support, earthquake shaking, vibration.
• Decrease of resisting force: rise in pore water pressure (rain, snow melt, reservoir drawdown), weathering, loss of cohesion, removal of vegetation, clay softening, joint opening.

Steps

1. Slope is made unstable by preparatory factors (weak rock, steep slope, adverse structure).
2. A trigger (heavy rain, earthquake, cutting) acts.
3. Cracks and tension cracks form at the crown.
4. Slip surface develops and the mass moves downward by sliding, falling, toppling or flowing.
5. Material stops at gentle slope and forms the deposit.

A Glacial Lake Outburst Flood (GLOF) is the sudden release of a large volume of water from a glacial lake, usually dammed by a moraine or ice, which produces a destructive flood and debris flow downstream.

How it occurs

1. Glaciers retreat due to climate warming, leaving meltwater in depressions behind end or lateral moraines.
2. The lake grows (e.g. Imja, Tsho Rolpa) and the unconsolidated moraine dam holds it.
3. A trigger acts and the dam breaches, by overtopping, piping or collapse.
4. A flood wave with high discharge, rock and ice debris rushes down the valley.

 glacier --> [ lake ] |moraine dam|
                      v breach
             ======> flood + debris ===>

Causes

• Climate change and glacier melt giving rapid lake growth.
• Ice or rock avalanche or landslide entering the lake, producing waves that overtop the dam.
• Melting of ice core in the moraine; piping and seepage.
• Earthquake shaking.
• Heavy rainfall or snow melt raising the lake level.
• Weak, steep, unconsolidated moraine dam.
• Failure of an upstream lake (cascade).

Consequences in Nepal

• Loss of life and livestock; the 1985 Dig Tsho GLOF destroyed the Namche hydropower project, bridges and farmland.
• Damage to hydropower plants, roads, trails, bridges and settlements; the 2016 Bhote Koshi and the 2021 events caused heavy damage.
• Heavy erosion and sedimentation, change of river course, and damage to farmland far downstream, sometimes more than 100 km.
• Trans-boundary effects (Tibet to Nepal to India).
• Mitigation: monitoring, early warning, artificial lowering of the lake (Tsho Rolpa), spillway, and land-use planning.

An earthquake is the sudden shaking of the ground caused by the release of strain energy stored in the earth's crust, which travels as seismic waves.

How an earthquake occurs

1. The crust is divided into tectonic plates which move slowly (cm per year) by mantle convection. India moves toward Eurasia by about 4-5 cm/yr.
2. At plate boundaries and faults, rocks are locked by friction, so strain accumulates and the rocks deform elastically.
3. When the stress exceeds the strength of the rock or the friction on the fault, the rocks suddenly slip.
4. The stored elastic energy is released as seismic waves from the focus (hypocentre); the point on the surface above it is the epicentre.

Causes: tectonic (most important), volcanic, reservoir-induced, mining and explosions.

Elastic rebound theory (H. F. Reid, 1906, after the San Francisco earthquake)

1. Rocks on both sides of a fault are strained slowly by plate motion; they bend and store elastic energy like a spring.
2. When the strain exceeds the rock strength, rupture occurs at the weakest point.
3. The rocks spring back (rebound) to a less strained position, and the fault moves; the energy is released as seismic waves.
4. The process then repeats, which is why earthquakes recur.

 (a) original   (b) strained   (c) after rupture
  |  |  |         \  \  \          |  |  |
  |  |  |         \  \  \         --|--|--   offset

A landslide is the downslope movement of a mass of rock, debris or earth under gravity. It occurs when the shear stress exceeds the shear strength.

Causes

1. Natural

• Steep slope and high relief; weak rocks (phyllite, shale, Siwalik mudstone), weathered rock and thick colluvium.
• Adverse geological structure: joints, bedding or foliation dipping out of the slope, faults and shear zones.
• Heavy rainfall, snow melt, and high groundwater (pore pressure).
• Earthquake shaking.
• River undercutting and toe erosion.
• Weathering and loss of vegetation.

2. Human (anthropogenic)

• Unplanned road cutting and slope excavation.
• Blasting, quarrying and mining.
• Deforestation and overgrazing.
• Irrigation and leakage of water, poor drainage.
• Loading by buildings and fills; reservoir fluctuation.

Mitigation (preventive and control) measures

1. Avoidance: hazard mapping and zoning; avoid building on unstable slopes.
2. Drainage: surface drains, catch drains, sub-surface drains and horizontal drains to reduce water.
3. Slope geometry: reduce slope angle, benching, removal of loose material at the head; add weight at the toe.
4. Retaining structures: retaining and breast walls, gabions, crib walls at the toe.
5. Reinforcement: rock bolts, anchors, soil nailing, wire mesh, shotcrete.
6. Bio-engineering: grass, shrubs and trees to bind the soil.
7. River training: spurs and check dams to prevent toe erosion.
8. Monitoring and early warning: inclinometers, crack gauges, rain gauges.
9. Community awareness and land use control.

A hazard is a natural or man-made event, process or condition with the potential to cause loss of life, property or environmental damage. Risk is the expected loss (probability of harm) from a hazard to the elements exposed, taking account of their vulnerability.

$$\text{Risk} = \text{Hazard} \times \text{Vulnerability} \times \text{Exposure}$$

Example: an earthquake in a desert is a hazard but has little risk; the same earthquake in Kathmandu is a high risk.

Seismic waves are the elastic waves produced by an earthquake that carry energy through and over the earth. They are of two main groups.

A. Body waves (travel through the interior)

1. P-waves (primary, compressional): particles vibrate parallel to the direction of travel (push-pull); fastest (about 5-8 km/s in crust); travel through solids, liquids and gases; arrive first.
2. S-waves (secondary, shear): particles move at right angles to the direction of travel; slower (about 3-4.5 km/s); travel through solids only, not liquids (so they showed the outer core is liquid); arrive second; more damaging than P-waves.

B. Surface waves (travel along the surface)

3. Love waves (L): horizontal shear motion, no vertical movement; fast surface wave.
4. Rayleigh waves (R): elliptical rolling motion like ocean waves; slowest, but largest amplitude and most destructive.

Difference between P- and S-waves

The S-P time lag at a station gives distance to the epicentre, and three stations locate it by triangulation.

Structural control means that the orientation and nature of geological structures (bedding, foliation, joints, faults, folds) decides whether a slope will fail, and in which manner.

1. Dip of bedding/foliation versus slope:
   • Dip slope (dip direction same as slope, dip less than slope angle): bedding planes daylight, planar slide.
   • Dip into the slope (reverse): stable, but toppling is possible.
   • Dip across the slope: wedge or lateral movement.
2. Joint sets: two intersecting joint sets form a wedge whose line of intersection daylights in the slope face (wedge failure).
3. Faults, shear zones and thrusts: crushed, clay-rich weak zones act as slip surfaces and conduits for water; most large slides are located on the MBT and MCT zones.
4. Folds: limbs dipping out of the slope are unstable; fold hinges have fractured rock.
5. Joint spacing and persistence: closely spaced, continuous joints give rock fall and toppling.
6. Weak interbeds (clay, mica schist, shale layers) form planes of sliding.
7. Discontinuity condition: smooth, clay-filled, wet joints with low friction.

Conditions for planar failure: strike of the plane within about 20 degrees of the slope face, dip of plane less than the slope angle, and more than the friction angle. These are checked by stereographic projection (kinematic analysis).

Hazard

A hazard is a potentially damaging natural or man-made event or phenomenon (such as earthquake, landslide, flood) that may cause loss of life, injury, property damage or environmental harm. It is described by its magnitude, location, frequency and probability.

Vulnerability (danger)

Vulnerability is the degree of loss or damage likely to occur to a given element (people, buildings, roads) when exposed to a hazard of given intensity. It is expressed from 0 (no loss) to 1 (total loss). It depends on construction quality, poverty, preparedness and location.

Risk

Risk is the expected loss (lives, injuries, property, economic activity) caused by a hazard in a given area and time. It combines the probability of the hazard, the elements at risk and their vulnerability.

$$\text{Risk} = \text{Hazard} \times \text{Vulnerability} \times \text{Elements at risk}$$

Example: a landslide on a steep slope is a hazard; the weak houses below are vulnerable; the possible loss of those houses and lives is the risk.

Main geological hazards

1. Earthquakes (and tsunami, surface rupture, liquefaction).
2. Landslides, rock falls and other mass movements including debris flow.
3. Floods, GLOF and LDOF.
4. Soil erosion and gully/bank erosion.
5. Volcanic eruptions.
6. Subsidence and sinkholes (karst, mining, groundwater withdrawal).
7. Swelling and collapsing soils.
8. Snow and ice avalanche.

Difference from man-made hazards

Note that human acts (deforestation, road cutting) can trigger geohazards such as landslides.

A geological hazard (geohazard) is a geological condition, process or event that is dangerous or potentially harmful to people, property and the environment. It arises from the earth's internal (endogenic) or external (exogenic) processes.

Types

1. Internal (endogenic) processes:
   • Earthquake and related ground rupture, liquefaction, tsunami.
   • Volcanic eruption.
2. External (exogenic) processes:
   • Mass movements: landslide, rock fall, debris flow, creep, avalanche.
   • Flood, GLOF, LDOF.
   • Erosion (soil, gully, river bank) and sedimentation.
3. Ground-related hazards: subsidence, sinkholes, swelling and collapsible soil, expansive clay.
4. Glacier and snow hazards: glacier surge, snow avalanche, permafrost thaw.

Nepal lies in the central Himalayan seismic belt, one of the most seismically active regions in the world, because of the following.

1. Plate tectonics: the Indian plate collides with and underthrusts the Eurasian plate at about 4-5 cm per year (around 2 cm per year across the Himalaya), so large strain accumulates on the Main Himalayan Thrust and is released as earthquakes.
2. Active thrusts and faults: MFT, MBT, MCT and other active faults cross Nepal from east to west.
3. Historical earthquakes: 1255, 1344, 1408, 1833, 1934 Bihar-Nepal (M 8.1) and 1988 Udayapur (M 6.8), 2015 Gorkha (M 7.8) and Dolakha (M 7.3) earthquakes, killing thousands.
4. Seismic gap: the western Nepal region has not had a great earthquake for a long time, so strain is stored there.
5. Frequent small earthquakes are recorded daily by the National Seismological Centre.
6. Hazard zoning: the Nepal National Building Code (NBC 105) puts the whole country in a high seismic zone, with Z about 0.35-0.4 g for most of it.
7. Geological conditions: soft sediments in the Kathmandu basin amplify shaking.

Soil erosion is the detachment and removal of soil particles by water, wind, ice or gravity.

Types of soil erosion

1. Splash (raindrop) erosion: impact of raindrops detaches particles from bare soil.
2. Sheet erosion: thin uniform layer of topsoil removed by surface run-off over the whole slope; hard to notice.
3. Rill erosion: small, narrow channels (a few cm deep) formed by concentrated flow; can be removed by ploughing.
4. Gully erosion: deepened rills, widened into deep channels that cannot be crossed by machinery; common on Siwalik and loose soils.
5. Stream (river) bank erosion: cutting of banks by flow and undercutting of the toe.
6. Wind erosion: in dry, bare land.
7. Tunnel (piping) erosion: sub-surface flow carries particles, forming pipes.
8. Glacial erosion by moving ice.

Types of mass movement

Fall, topple, slide (rotational, translational), spread, flow (debris flow, mud flow), creep and complex movement.

Control: vegetation, terracing, check dams, drainage, contour farming, bio-engineering.

A landslide has the following parts.

   crown   main scarp
     |  _______
     | /   head \ 
 ----+/  ___      \ minor scarp
        /   \ body  \
       / slip surface\
      /  (rupture)    \
     /_______ foot_____\ toe
                 deposit  ~~~ river

• Crown: undisturbed material above the main scarp.
• Main scarp: steep surface at the upper edge of the slide, caused by movement of the mass away from stable ground.
• Minor scarp: secondary steps within the moved mass.
• Head: upper part of the moved mass, along the contact with the main scarp.
• Body (main body): the displaced material lying on the rupture surface.
• Surface of rupture (slip surface): the surface along which sliding takes place.
• Foot: the moved mass beyond the toe of the rupture surface.
• Toe: the lowest edge of the displaced mass, farthest from the main scarp.
• Flanks: the sides of the slide.
• Tension cracks at the crown; transverse cracks and pressure ridges in the body and toe.

The factors of mass movement are those that increase the shear stress or lower the shear strength of the slope.

Causes by group

1. Gravity and slope: steep slope angle, large relief.
2. Water: rainfall, snow melt and groundwater raise pore pressure and weight, and reduce friction and cohesion.
3. Material: weak, weathered or clay-rich rocks and soils; loose debris.
4. Structure: joints, bedding and foliation dipping out of the slope, faults and shear zones.
5. Vegetation: removal of forest lowers root cohesion.
6. Earthquake and vibration: shaking, blasting, traffic.
7. Toe undercutting: by river erosion or road cuts.
8. Loading: buildings, fills, and water in the reservoir.
9. Weathering: freeze-thaw, wetting-drying, chemical decay.
10. Human activities: mining, irrigation, deforestation.

Mitigation means reducing the effect and the risk of hazards. Measures are structural (engineering) and non-structural.

Non-structural

1. Hazard and risk mapping and zoning; land-use planning to avoid dangerous areas.
2. Monitoring and early warning: seismographs, rain and stream gauges, GLOF alarms, inclinometers.
3. Building codes (NBC) and enforcement.
4. Education, awareness and preparedness: drills, evacuation plans, emergency funds and insurance.
5. Afforestation and control of deforestation and unplanned construction.

Structural

1. Landslides: drainage, slope re-grading, retaining walls, rock bolts, shotcrete, bio-engineering.
2. Floods and erosion: embankments, spurs, check dams, river training, reservoir storage.
3. GLOF: lowering of lake level by drainage channels or siphon, spillways, protective dams.
4. Earthquakes: earthquake resistant design, base isolation, retrofitting, avoiding active faults.
5. Subsidence: grouting, controlled groundwater pumping.

Major geological hazards of the Himalaya

1. Earthquakes from active thrusts (MFT, MBT, MCT); 2015 Gorkha earthquake.
2. Landslides, rock fall and debris flow on steep slopes of weak rocks during the monsoon.
3. Floods and flash floods; river bank erosion and sedimentation (Terai).
4. GLOF and LDOF in the high mountains.
5. Snow avalanche and glacier hazards.
6. Soil erosion and gullying in Siwalik.
7. Subsidence and karst in limestone areas.

Mitigation measures

1. Hazard mapping, zoning and land-use control; avoid dangerous locations.
2. Landslide control: drainage, slope grading, retaining walls, rock bolts, bio-engineering, and check dams for debris flow.
3. Flood control: embankment, spurs, river training, early warning.
4. GLOF: monitoring, lake-level lowering, early warning systems.
5. Earthquake resistance design following NBC, retrofitting, avoiding active faults.
6. Afforestation, watershed management and controlled road construction.
7. Community awareness, disaster preparedness, and monitoring.

The Nepal Himalaya is very prone to landslides due to a combination of geological, topographic, climatic and human factors.

1. Young, active tectonics: The Himalaya is still rising due to the Indo-Eurasian collision (about 4-5 cm/year), producing steep, unstable slopes and frequent earthquakes that trigger slides.
2. Steep topography: very high relief and steep slopes; rivers cut deep gorges and undercut the toe.
3. Weak and fractured rocks: phyllite, schist, slate, shale and Siwalik mudstone; rocks are folded, faulted and thoroughly jointed, especially near MBT and MCT.
4. Adverse structure: foliation and joints dipping out of the slope.
5. Intense monsoon rainfall: 80% of annual rainfall in June-September, with cloudbursts, increasing pore water pressure and erosion.
6. Weathering: deep weathering and thick colluvium and residual soil.
7. Glaciers and snow melt in high areas.
8. Human activity: unplanned road construction by bulldozer, deforestation, agriculture on steep slopes, quarrying and settlements.
9. Earthquakes (2015) produced thousands of landslides.

Classification of engineering evaluation of hazard

Hazard can be evaluated in two ways.

1. Absolute hazard evaluation: the hazard is given a numerical value of probability (for example, a 1 in 100 year event) or a quantity such as factor of safety, annual probability, expected displacement, magnitude. It is quantitative and used in design.
2. Relative hazard evaluation: areas are ranked as high, moderate or low by comparison with each other, without absolute probability; qualitative or semi-quantitative (hazard zoning maps).

Absolute hazard assessment (how it is done)

1. Collect data: past events (inventory), geology, slope, rainfall, earthquake records, geotechnical parameters.
2. Deterministic method: compute the factor of safety of the slope, for example

$$F = \frac{cA + W\cos\beta\tan\phi}{W\sin\beta}$$

with $F<1$ meaning failure. 3. Probabilistic method: use frequency of past events to calculate return period $T$ and annual probability $P = 1/T$; probability of occurrence in $n$ years is $1-(1-1/T)^n$. 4. Define the design event (design earthquake, 100-year flood) and compute magnitude, runout distance or displacement. 5. Present results as hazard maps with numerical values and use them in design and risk calculation.

Relative hazard assessment ranks the different parts of a region according to their degree of hazard, relative to one another, without giving the exact probability or time of occurrence. It answers "which area is more hazardous?" rather than "how likely is the event?".

Method

1. Choose the causative factors (slope, lithology, structure, land use, rainfall, distance to faults and streams, landslide inventory).
2. Give each factor class a rating or weight (rating of low, moderate, high).
3. Overlay the maps (GIS) and add the weighted values to give a hazard index.
4. Divide the region into zones: very low, low, moderate, high, very high.

Uses

• Regional planning and land-use zoning.
• Priority for detailed investigation and mitigation.
• Quick, cheap hazard maps when data are limited.

It is qualitative or semi-quantitative, subjective, and not suitable for exact design.

The engineering geological factors that cause hazards in the Nepal Himalaya are the following.

1. Active tectonics: continuous collision of Indian and Eurasian plates; active thrusts (MFT, MBT, MCT) cause earthquakes, uplift and fractured rock.
2. Lithology: weak, soft rocks such as phyllite, schist, shale, Siwalik mudstone and loose alluvium; soluble rocks (limestone) with cavities.
3. Geological structure: folds, faults, joints and foliation with adverse orientation; shear zones with crushed rock.
4. Steep slopes and high relief, with deep river valleys and strong river undercutting.
5. Weathering and thick unconsolidated soil cover, colluvium, glacial moraine and talus.
6. Climate: intense monsoon rain, snow melt, freeze-thaw; glacier retreat and glacial lakes.
7. Hydrogeology: high groundwater and pore water pressure, springs and seepage.
8. Seismicity, shaking and liquefaction of loose saturated sand in the Terai and Kathmandu basin.
9. High erosion and sediment transport by rivers; river shifting in the Terai.
10. Human influence: unplanned roads, deforestation, settlements on unstable ground and quarrying.

A landslide dam outburst flood (LDOF) is the sudden flood produced by the failure of a natural dam formed when a landslide blocks a river.

How it occurs

1. A large landslide, rock avalanche or debris flow (triggered by heavy rain or earthquake) falls into a narrow river valley and blocks the river, forming a dam.
2. Water accumulates upstream and forms a lake.
3. The dam is made of loose, unsorted, poorly compacted material and has no spillway.
4. The dam fails by overtopping and erosion, piping/seepage, or sudden slope collapse. Failure may occur within minutes to months, sometimes years.
5. A large volume of water and debris is released as a destructive flood wave that travels downstream and may erode and enlarge itself.

   landslide
      \\  
  =====\\|dam|------> breach
  river  [lake]         flood wave

Examples: 2014 Jure (Sunkoshi) landslide dam and the 2015 events after Gorkha earthquake.

A flood is the overflow of a river onto normally dry land, mostly caused by prolonged or intense rainfall (monsoon) or snowmelt. A GLOF (Glacial Lake Outburst Flood) is a sudden release of a large volume of water from a glacial lake when its moraine or ice dam fails.

Control: floods are controlled by embankments, spurs, dams and early warning. GLOF hazard is reduced by lowering lake level (siphons, tunnels, open channels), strengthening the moraine dam and installing monitoring and warning systems (e.g. Tsho Rolpa).

Nepal lies on the Himalayan collision zone where the Indian plate underthrusts the Eurasian plate at about 40 to 50 mm per year, so earthquakes are frequent and severe (1934 Nepal-Bihar M 8.0 to 8.4, 1988 Udayapur M 6.6, 2015 Gorkha M 7.8). The main effects are:

1. Ground shaking: the main cause of damage. Collapse of unreinforced masonry, mud-mortar and stone houses; about 9,000 deaths and over 500,000 houses destroyed in 2015.
2. Landslides and rockfalls: shaking triggers thousands of slides on steep, fractured Himalayan slopes (Langtang, Sindhupalchok, Gorkha), blocking roads and trails.
3. Liquefaction and ground failure: in loose, saturated sands (Kathmandu valley fill, Terai) the soil loses strength, causing settlement, tilting of buildings and sand boils.
4. Surface rupture and ground cracks: along faults such as the Main Himalayan Thrust and its branches.
5. Damming and floods: landslides block rivers and form temporary lakes, with the danger of outburst floods. Cracks and damage to dams and canals.
6. Snow and ice avalanches: the 2015 shock caused avalanches at Everest Base Camp and Langtang.
7. Damage to infrastructure and heritage: roads, bridges, water supply, hydropower plants, temples and monuments (Dharahara, Durbar Squares).
8. Secondary effects: fires, epidemics, loss of livelihoods, displacement and heavy economic loss (about 7 billion US dollars in 2015).

Reduction: seismic zoning, Nepal Building Code (NBC 105), retrofitting, safe site selection and public awareness.

Seismicity is the frequency, distribution and magnitude of earthquakes occurring in a region over a period of time. It shows how active an area is and is the basis of seismic hazard assessment.

• Earthquakes are measured by magnitude (energy released, Richter or moment magnitude) and intensity (effects at a place, Modified Mercalli scale).
• Seismicity follows the Gutenberg-Richter relation $\log N = a - bM$, where $N$ is the number of events with magnitude $\geq M$.
• Seismicity is shown on epicentre maps and seismic zoning maps. Most earthquakes occur along plate boundaries and active faults.

Nepal: the Himalaya is one of the most seismically active belts because of the collision of the Indian and Eurasian plates. The activity is concentrated along the Main Central Thrust, Main Boundary Thrust and Main Frontal Thrust. Great events in 1255, 1344, 1408, 1833, 1934 and 2015 show a repeat cycle, and the central Himalaya is regarded as having a seismic gap that may still hold stored strain.

Seismicity data are used to design earthquake-resistant structures (NBC 105) and to select safe sites for dams, tunnels and cities.

Erosion is the detachment and removal of soil and rock by water, wind or ice. In Nepal, water erosion on steep, cultivated and deforested slopes and river bank erosion are the main problems. The mitigation measures are:

1. Vegetative (bio-engineering) measures: afforestation, grassing, shrub planting, brush layering and live fascines to bind soil and reduce raindrop impact.
2. Agronomic and land-use measures: contour farming, mulching, crop rotation, strip cropping, controlled grazing and avoiding cultivation on steep slopes.
3. Terracing: bench terraces and bunds to reduce slope length and runoff velocity.
4. Drainage control: surface drains, cut-off drains, diversion channels and energy dissipaters to carry runoff safely off the slope.
5. Structural measures: check dams in gullies, gabion walls, retaining walls and sub-surface drains.
6. River bank protection: riprap, gabion revetments, spurs/groynes, embankments and bank vegetation.
7. Slope grading: cutting stable slope angles and covering bare slopes with geotextile or jute netting.
8. Awareness and regulation: community forest management and watershed management.